MEMS technology involves the process of designing and building micro-sized mechanical and/or electrical structures with technology generally developed for 5V CMOS processes common to IC fabrication. In one area of MEMS device fabrication, typically referred to as surface micro-machining, layers of semiconductor, metal, and insulator materials are utilized to build structures which can be activated by electrostatic, electromagnetic, thermal, or pneumatic means, among others. A balancing force to these externally imposed forces is often provided by the mechanical properties of the structures, such as the spring force in deflected beams, bridges, or membrances. Such MEMS devices are projected to be used in areas of biomedical engineering, aerospace, automotive, data storage, or optical telecommunications, where they are used as dispensers, sensors, actuators, read/write heads, or optical signal processing.
FIGS. 1 and 2 illustrate cross sectional views of an embodiment of a grating light valve™ light modulator with movable ribbons. The substrate 119 comprises a silicon layer 120 and a passivating layer 122, such as silicon dioxide. A conducting layer 124 is configured to receive a charge or to be held at ground potential. FIG. 1 illustrates the ribbon 134 in an undeflected state. FIG. 2 illustrates a ribbon 134 in a deflected state. Deflection is typically induced in a ribbon layer 134 by applying a voltage potential to the ribbon 134 with respect to the conducting layer 124, typically by means of a controller circuit.
According to the embodiment illustrated in FIG. 1, the conducting layer 124 is formed on top of the passivating layer 122. The ribbon 134 bridges the conducting layer 124, with an air gap 132 separating the ribbon 134 from the conducting layer 124. Referring to FIGS. 1 and 2, the ribbon 134 comprising a resilient layer 126 which lends tension, flexibility and elasticity to the ribbon structure 134, allowing the ribbon 134 to return to its original position when a deflecting force is removed. The resilient layer 126, also known as the ribbon layer, is typically a stoichiometric silicon nitride layer such as Si3N4. Tension inherent in the silicon nitride film provides the restoring force to the applied potential force. The resilient layer 126 is typically on the order of about 50-150 nanometers thick in conventional embodiments. The second layer in the ribbon 134, layer 128, is a layer which balances the lateral stress between the nitride layer 126 and the aluminum layer 130, such that in the case of wide ribbons, the curvature after ribbon release is minimized. This layer 128 typically consists of silicon di-oxide, and will be absent for narrow ribbons. Typical oxide layer thicknesses are on the order of about 800 to 2000 nm. The third layer of the ribbon 134 is the aluminum reflecting layer 130, which is deposited against the surface of the silicon oxide 128, or with the absence of the oxide, against the surface of the ribbon nitride 126. The aluminum reflecting layer 130 functions to reflect light for various applications of the grating light valve™ light modulator. The aluminum layer 130 also functions as the complementary capacitor plate, and thus is the electrode that forms one half of the structure across which the field is applied. The aluminum layer 130 is typicaly between about 650 and 1500 nm thick in the conventional embodiments. As noted, an air gap 132 separates the substrate 119 from the ribbon 134. As can be seen in FIGS. 3 and 4, the ribbon layer 126 is bonded to the substrate 122 at an end connection point 135 and/or at a center anchor 136.
The deformable ribbons 134 of grating light valve™ light modulators are representative of a feature common to some MEMS devices. Because most MEMS devices are partly mechanical in nature, they typically involve an electrically or thermally induced mechanical motion of some sort. Moreover, mechanical motion within MEMS devices typically causes elastic material deformation, as illustrated by the ribbons in FIGS. 1 and 2.
The ribbon 134 is fabricated to exhibit an inherent tension defining a natural resonant frequency, and requiring a specific force necessary to deflect the ribbon 134 relative to the substrate surface 119, as illustrated in FIG. 2. Static equilibrium is maintained as the electrostatic force between ribbon 134 and substrate 124 is balanced by the tensile force in ribbon 134. The force between the ribbon 134 and the substrate 119 is transmitted through the end connection point 135 and the center anchor 136 according to the embodiment shown in FIG. 1.
The voltage required to fully deflect the ribbon, known as the switching voltage or pull-down voltage, is typically on the order of about 15-25 volts in certain conventional embodiments. However, the tension within the ribbon across the substrate does not remain constant over a range of temperatures. It typically reduces when the temperature increases and increases as the temperatures decreases. This has a variety of undesirable effects, one of which is that the changing tension causes the pull-down voltage required to fully deflect the ribbon to change as the temperature changes. The fundamental resonance frequency which depends on ribbon characteristics also changes as the tension changes over a range of temperatures. Because damping time is largely a function of the ribbon mass, the damping time remains largely constant in spite of temperature changes, and is typically in the range of about 0-10 μsec in conventional approaches. There is therefore a desire for a method and apparatus for athermalizing a MEMS design to achieve a constancy of operation over an operational temperature range. More specifically, a desire exists for a method and apparatus for athermalizing the ribbon of a grating light valve™ light modulator to maintain a constancy of tension over an operational temperature range. There is further a desire for a method and apparatus for leveling the deflection voltage of a MEMS device over an operational temperature range. Additionally, there is a desire for leveling the resonant frequency of a MEMS device over an operational temperature range.